Preferred contact materials for commercial applications must be resistant to both erosion and welding inasmuch as arc temperatures in an electrical contact environment can approach several thousand degrees centigrade. It was thus realized early on that fine silver needed to be strengthened in order to be suitable for such demanding applications. One of the first means employed, and still in use today, was (and is) oxide-dispersion strengthening with cadmium oxide. Silver-cadmium oxide contact materials have been fabricated by a variety of methods.
One method involves the preparation of a silver and cadmium oxide powder mixture followed by sequentially pressing, sintering and extruding the resulting material to form a contact material. Generally speaking, such contact materials include from about 5 to about 20 percent cadmium oxide dispersed in a silver matrix. The sintered material possesses excellent anti-weld properties, but the erosion resistance of such contact materials is only considered fair. This latter drawback is probably due to the fact that materials prepared as above typically contain up'to approximately two percent void space.
As an alternative to the above method of forming a silver-cadmium oxide contact material, it has been found that alloying fine silver with about 13.5 percent by weight of the total mixture cadmium oxide followed by: (a) drawing the alloy into a wire; (b) oxidation of the cadmium in the wire by internal oxidation; (c) chopping the wire into granules; (d) compacting pressing and sintering, the granules into ingot; and finally, (e) extruding the sintered wire into a shaped body; produces a contact material possessing both good erosion and anti-weld characteristics.
Generally speaking, it is possible to internally oxidize certain components in silver alloy sheets, such as cadmium (or indeed, a variety of metals) in a silver matrix since silver oxide decomposes at the lower temperatures required to oxidize certain other metals. Thus, it is possible to obtain a domain of other metal oxides disposed in a silver matrix by internally oxidizing an alloy by simply heating the alloy in an oxygen atmosphere at temperatures to form about 1400 to about 1600 degrees Fahrenheit (760.degree. to 870.degree. C.).
A third conventional method of producing silver-cadmium oxide contact material consists of alloying fine silver with about 13.5 percent cadmium, extruding the alloy or rolling it into a sheet, and oxidizing the sheet in a furnace at 1550.degree. F. (840.degree. C.) in an atmosphere consisting substantially of oxygen. The sheet thus produced also exhibits erosion-resistant and anti-weld properties, but it has been observed that when oxidized simultaneously from both sides, the sheet exhibits an oxide-depleted central region. This effect is believed to occur through a diffusion-type mechanism and produces undesirable non-uniformities in the final product.
With respect to the internal oxidation of a silver-cadmium sheet, it was proposed to weld two such sheets together along their interface in an airtight or hermetic fashion prior to internal oxidation and separating the sheets or slabs after internal oxidation was carried out. This method was found to produce a product which exhibited an oxide depleted layer at one surface (i.e. the internal interfacial surface between the formerly welded plates) which was, in any event, satisfactory, inasmuch as the single oxide-depleted surface could be used for brazing or otherwise metallurgically affixing the material to a suitable mount. In this regard, fine silver is a preferable surface for brazing, soldering or like methods, as opposed to the oxide-dispersion strengthened materials typically used as contact materials. On the other hand, the outer surfaces of the silver cadmium oxide sheets so produced had uniform metal oxide domains and were thus most suitable for use as a contact material.
Concerns over the possible toxicity of silver/cadmium contact materials have led to formulation of alternate oxide-dispersion strengthened/silver matrix materials. U.S. Pat. Nos. 3,933,485 and 3,874,941, both to Shibata for example, disclose silver-tin-indium oxide materials for use as electrical contacts.
Silver-tin-indium oxide materials may be processed similarly to the more conventional contact materials described hereinabove, although they have slightly different processing characteristics. For example, if one processes a single sheet of silver-tin-indium sheet by simultaneously oxidizing both surfaces as noted above in connection with cadmium oxide, one obtains an oxide depleted central region, and fine silver exudes to the two opposed outer faces of the sheet (sometimes referred to as diffusion creep) resulting in two additional oxide depleted layers. This latter phenomenon does not appear to occur with silver/cadmium materials upon internal oxidation.
Exudation of pure silver produces drawbacks that the silver surfaces tend to tack-weld in the early stage of device operation, although one silver-rich surface may help achieve the desired surface properties for brazing.
It is known to internally oxidize a sheet or slab of silver-tin-indium coated with fine silver as described in U.S. Pat. No 4,647,322 to Shibata. According to the '322 patent, sheets of fine silver are roll-bonded to opposed surfaces of a silver-tin-indium slab and the resulting structure is then internally oxidized. Following oxidation of the tin and indium, the structure exhibits centrally located oxide depleted region. In order to produce a usable contact material, the slab is sliced or sawed along the central depleted region, thereby removing the oxide depleted layer and producing two sheets of contact material.
The foregoing method of Shibata has several drawbacks, including the cost associated with slicing or sawing the internally oxidized structure through its center and the criticality involved in the cutting process.
It is accordingly an object of the invention to produce more efficiently a silver-tin-indium oxide contact material.
It is another object of the invention to eliminate machining steps in the fabrication of such materials.
A further object of the invention is to produce a silver-tin-indium oxide contact material with at least one uniform dispersed oxide surface following internal oxidation.
Still further objects and advantages of the present invention will become readily apparent upon consideration of the following detailed description, figures and claims.